FIG. 1 is a simplified sectional view of an exemplary prior art n-channel MOS field-effect transistor 10 formed on a p-type semiconductor substrate 12. A n.sup.+ -type source 14 and a n.sup.+ -type drain 16 extend into substrate 12. A polysilicon gate 18 is positioned on a gate oxide layer 20 formed on substrate 12 between source 14 and drain 16. A field oxide layer 22 formed on substrate 12 encircles transistor 10 and electrically insulates it from adjacent devices on substrate 12. Typically, substrate 12 is primarily monocrystalline silicon, and gate oxide layer 20 is primarily silicon dioxide (SiO.sub.2).
Gate oxide layer 20 is typically formed by thermal oxidation of substrate 12 in a substantially pure oxygen atmosphere. In MOS VLSI circuits, however, such gate oxide layers 20 can exhibit undesirable characteristics such as, for example, relatively high defect densities and charge trapping, and relatively low reliability and resistance to hot-carrier effects (e.g., variations in transconductance and threshold voltage caused by hot-carrier stressing).
Improved gate oxide layers 20 have been formed by rapid thermal processing of substrate 12 in a nitrous oxide (N.sub.2 O) atmosphere. These improved gate oxide layers 20 are highly resistant to hot-carrier effects and exhibit reduced electron trapping, but have unacceptable thickness and compositional nonuniformities. Thickness and compositional uniformity are improved when gate oxide layers 20 are grown in a nitrous oxide atmosphere using conventional thermal processes. For example, gate oxide layers 20 with thicknesses of up to about 6.5 nm have been grown at temperatures between 950.degree. C. and 1000.degree. C.
Conventional thermal growth of silicon dioxide in a nitrous oxide atmosphere occurs at a relatively low rate and is self-limiting at temperatures in the 950.degree. C.-1000.degree. C. range. As a consequence, an increased oxidation period at these temperatures is incapable of growing an oxide layer to a thickness greater than about 6.5 nm, thereby requiring higher oxidation temperatures to grow thicker layers. For example, a gate oxide layer 20 with a thickness of 12 nm can be formed in nitrous oxide at an oxidation temperature of 1057.degree. C. over a period of about 180 minutes.
A gate oxide layer 20 with a thickness of about 12 nm is advantageous in, for example, 64 Mbit dynamic random-access memory (DRAM) integrated circuits. However, transistors 10 with gate oxide layers 20 formed in a nitrous oxide atmosphere at a temperature of 1057.degree. C. have severe junction leakage characteristics. As a consequence, these gate oxide layers 20 are unacceptable for MOS semiconductor devices such as 64 Mbit DRAM integrated circuits.